The invention relates to devices for providing protective coatings on glass optical waveguide fibers.
Typically, optical waveguide fibers are produced by first manufacturing a glass preform and then drawing the preform into an optical fiber. The fiber can be drawn by placing an end of the preform into a hot furnace to soften the end thereof. The preform is then slowly fed into the furnace as an optical fiber is drawn out of the furnace from the softened end.
Optical waveguide fibers drawn from glass preforms in the manner described above are generally quite strong. For subsequent handling of the fibers, for example in producing optical fiber cables, it is necessary to maintain this high strength. However, subsequent handling can produce surface defects which significantly reduce the strength of the fiber. In order to reduce or eliminate surface defects during subsequent handling, one or more protective coatings are typically applied to the fibers immediately after they are drawn, and before they are wound on spools.
A protective coating may be provided on an optical fiber by a coating applicator device. The coating applicator may comprise, for example, a pair of wire-coating dies mounted in line in a holding block. A coating material is supplied under pressure between the two dies. As the fiber passes through the coating material, a coating is applied. Excess coating material is sheared off by the exit coating die.
While the optical fiber is provided with a protective coating after it is drawn from the hot furnace and before it is wound on a spool, the protective coating cannot be applied at any arbitrarily chosen time between these two production steps. As the fiber is drawn from the hot furnace, it must be allowed to cool for a predetermined amount of time before the coating or coatings are applied. If the fiber temperature is too high when the coating is applied, the coating material can be damaged by the excess heat.
In order to allow the hot fiber to cool before it is coated, the coating applicator must be fixed at a distance, d, from the furnace. The distance, d, is a function of the necessary cooling time, t, and the drawing velocity, v, of the fiber being drawn. This distance is given by d=vt. Thus, increasing the drawing velocity or the necessary cooling time increases the required distance between the furnace and the coating applicator. Since the fiber is drawn vertically down out of the furnace in a structure called a draw tower, increasing the draw velocity or the cooling time calls for increasing the height of the draw tower.
If two or more coatings are to be applied to an optical fiber, each coating is applied by a separate coating applicator, and is then cured before the next coating is applied. When two or more coating applicators are used, they are thus separated by a distance sufficient to permit curing of the previous coating before the application of the subsequent coating. The height of the draw tower, in such instances must then be increased by the additional curing distance plus the length of the second coating device. Alternatively, the draw velocity can be reduced to allow the first coating device to be placed closer to the furnace by this distance, while still allowing adequate time for the fiber to cool before the first coating is applied.